first, it depends on WHY you are using the spur. If you need a spur to get the horse forward, than imo the horse's training needs to be addressed, as you want a horse that is sensitive to the natural aids and moves forward willingly.
Next, if you are using the spur to refine your queues, make sure you are ready. Your leg should be stable both with and without stirrups (so as to prevent from accidentally using the spur) and the horse must be trained to spur aids. Getting on a horse that doesn't know refined aids with a spur and asking for a shoulder in can result in a lurch forward and nervous horse if the horse isn't ready yet.
I use spurs on my eventer clyde cross when we're doing focused dressage work as well as at shows. In dressage I use it to refine the leg aids and let him know more specifically where and what body part I want him to bend. Xc I use the spur if he gets stuck to my leg on approach, and use both spurs evenly to indicate "GO Forward!" as necessary. When you get even more refined, you can use the spur to tell a horse how and where to jump - such as "jump long" or "jump high" or "takeoff here". These are effective queues to have when getting more competitive in the jumper ring especially. I use those queues with my OTTB however due to having VERY sensitive skin, I can't ride him in spurs much (if at all). The training is there though and he's learned to respond just to my leg (which does offer slightly less refinement) based on how I want him to jump. Especially handy out field hunting!
As an aside - also watch for skin sensitivity as well as spur marks. You should never leave marks on your horse from your spurs and if you do, you need to go back to working on your leg. In some rare cases, horses can also be sensitive to the metal of the spurs and break out in hives or lose fur like my OTTB. He's a rare case though as he will even lose some fur in reaction to his own sweat if he sweats too much (and a girth without the right amount of elastic will give him edemas under the armpits....he's a special case LOL).
Most importantly, before using spurs, make sure both YOU and your horse are ready and know how and when and where to use them! It's also always best to start small rather than jumping right in with swan neck or 1/2" spurs.